Architecture and management of a geological repository - Andra

• Monitoring the ventilation airMonitoring the air flow at the cell inlets and outlets enables us to complete the heat balance for thecell, to determine the hygrometry of the atmosphere and at a later date to detect any hydrogen orradioactive gases. The air flow can be easily measured in the return air ventilation duct, while the airtemperatures are measured, at the inlet and outlet, with vibrating wire sensors. The humidity sensors(hygrometer), which require regular maintenance, can be installed in the drift and in the air exhaustduct to measure the relative humidity of the inlet and outlet air. Finally, additional measurements canbe carried out by means of cell inlet and outlet air sampling systems.10.3.7.2 The evolution of the module B monitoring system after the packages are put in placeThe monitoring system described above is put in place during construction in order to monitor theevolution of the structure for as long as possible during the repository operation and mainly during andafter the packages are put in place. The strength and the redundancy of the monitoring equipmentenvisaged provides a reasonable guarantee that this system will continue to provide reliable data for asufficient period to allow us to assess the behaviour of the cell during the stage "after the packages areput in place".The environmental constraints affecting the sensors are moderate. The temperature expected at thesensors should not exceed 50°C providing the cell is ventilated. The effect of radiation on theinstruments is reliable as most of these instruments are protected by the concrete lining. No loss ofequipment due to the environmental conditions is expected during this stage, except possibly thethermometers installed in the repository chamber wall.Based on the experience feedback, it is probable that approximately 90% of the pinpoint sensors, willremain operational for at least fifty years. However, the environmental conditions of sensors evolvevery little after twenty or thirty years and losses of this type of sensor occur mainly during the first fewyears of operation. A loss rate of less than 50% appears possible over a century and can be toleratedwithout causing a disturbance which would endanger the overall monitoring of the structure. It is truethat fiber optics sensors have much shorter term experience feedback, (of the order of around tenyears), but they do not have characteristics which appear likely to render them fragile in the long term.The monitoring system, as it has been set up during the construction of the module can therefore beoperated during this stage for several decades as regards systems which can not be maintained(especially those in cells). Systems that can be maintained can be operated without any time limitation.10.3.7.3 The evolution of the module B monitoring system after the cell is sealedThe evolution of the environmental conditions brought about by the construction of the seal, has nosignificant consequences for the reliability of the sensors which have already operated without anypossibility of maintenance since the packages were put in place.However, the transmission of measurements carried out in the cell is rendered more complex by thepresence of the clay core. The need to maintain the hydraulic efficiency of the seal requires amodification in the transmission system. It is important to be able to extend monitoring beyond thismilestone in order to ascertain the conditions expected in the cell and the access drift if it is necessaryto reverse the process, and dismantle the seal or retrieve the packages.During a transitional period, the transmission of measurements through the seal is nevertheless apossibility, as long as it does not affect the long term mechanical and hydraulic efficiency of the seal.Several options are under study in this regard. They are founded on wireless transmission or ontemporary transmission by cables followed by repairs to the seal.Since it is not possible to monitor the resaturation, nor the swelling of the seal – which necessitatesseveral thousand years – seal monitoring will focus on the evolution of the rock in the near-field, thepossible evolution of the initial state (unsaturated) of the seal, and possibly the incipient resaturation inthe vicinity of the grooves.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM406/495